@Craig,
You are correct. AdMob doesn’t have their own rewarded video inventory. All such requests will be handed down to external SDK’s using their SDK-based mediation. I’d assume that the AdMob servers would be involved with the mediation process, however how the actual mediation occurs is something where we don’t have any insight. We don’t have any immediate plans to support AdMob SDK-based mediation at this point in time.
If you want to include mediation that also allows you to use your AdMob account, you can consider using Appodeal.